1. Field of the Invention
This invention relates to a laser beam generating apparatus wherein a secondary harmonic laser beam is generated by means of a non-linear optical crystal element.
2. Description of the Prior Art
A laser beam generating apparatus of the type mentioned has already been proposed by the inventor of the present application and is shown in FIG. 1. Referring to FIG. 1, a laser diode 1 such as a semiconductor laser element generates pumping laser beam, which is introduced into a convex lens 2. The pumping laser beam emerging from the lens 2 is introduced into a laser medium 5 by way of a concave mirror 3 and a quarter wavelength plate 4. Upon reception of such pumping laser beam, the laser medium 5, which may be, for example, Nd:YAG, forms a heat lens 5a and generates fundamental wave laser beam. The fundamental wave laser beam is introduced into a plane mirror 7 by way of a non-linear optical crystal element 6 of, for example, KTP. The fundamental wave laser beam is thus reflected by the plane mirror 7 and then introduced into the laser medium 5 again by way of the non-linear optical crystal element 6.
Then, the fundamental wave laser beam emerges in the leftward direction in FIG. 1 from the laser medium 5 and is introduced into the concave mirror 3 by way of the quarter wavelength plate 4. The fundamental wave laser beam is then reflected by the concave mirror 3 and introduced into the laser medium 5 again by way of the quarter wavelength plate 4. In this manner, the fundamental wave laser beam reciprocates between the concave mirror 3 and the plane mirror 7. Thus, an optical resonator 8 is constituted from the concave mirror 3, the quarter wavelength plate 4, the laser medium 5, the non-linear optical crystal element 6 and the plane mirror 7. The position where the fundamental wave laser beam reciprocates is concentrated by an action of the concave mirror 3. Consequently, the energy of the fundamental wave laser beam is amplified so that the KTP (KTiOPO.sub.4) generates secondary harmonic laser beam having a frequency equal to twice that of the fundamental wave laser beam due to phase matching of the type II. While the plane mirror 7 reflects almost all of the fundamental wave laser beam, it passes almost all of such secondary harmonic laser beam therethrough. As a result, a secondary harmonic laser beam is outputted from the optical resonator 8. (As regards phase matching of the type II, refer to, for example, U.S. Pat. No. 4,910,740).
The quarter wavelength plate 4, which is a double refracting element, is disposed such that, as seen in FIG. 2, the optical axis ne(4) thereof in the direction of an extraordinary ray may have a directional angle .theta. of 45 degrees with respect to the optical axis ne(6) of the non-linear optical crystal element 6 in the direction of an extraordinary ray. Since the directional angle .theta. of the quarter wavelength plate 4 is set to 45 degrees in this manner, a coupling phenomenon which may otherwise occur between two modes of the fundamental wave laser beam can be prevented, by which a secondary harmonic laser beam can be stabilized.
An incidence face 6a of the non-linear optical crystal element 6 for the fundamental wave laser beam is formed in an inclined relationship with respect to an optical axis LA1 of the fundamental wave laser beam. Since the incidence face 6a of the non-linear optical crystal element 6 is inclined with respect to the optical axis LA1 in this manner, the effective optical path length of the fundamental wave laser beam can be adjusted accurately to a predetermined value by adjusting the non-linear optical crystal element 6 in a direction perpendicular to the optical axis LA1 of the fundamental wave laser beam, that is, in a direction indicated by a double-sided arrow mark T in FIG. 1. The non-linear optical crystal element 6 is adjusted such that the double refraction amount thereof may be just equal to 90 degrees as a result of such adjustment.
The double refraction amount of the non-linear optical crystal element 6 is adjusted to just 90 degrees in this manner in order to achieve the following advantage. In particular, the fundamental wave laser beam emerging from the laser medium 5 is circularly polarized beam. The fundamental wave laser beam is changed into linearly polarized beam when it passes through the quarter wavelength plate 4. The fundamental wave laser beam in the form of linearly polarized beam is reflected by the concave mirror 3 and then passes through the quarter wavelength plate 4 again, whereupon it is changed back into circularly polarized beam. The fundamental wave laser beam passes through the laser medium 5 and is introduced into the non-linear optical crystal element 6 while it remains in the form of circularly polarized beam. Since the non-linear optical crystal element 6 is adjusted so that the double refraction amount thereof is accurately equal to 90 degrees, the fundamental wave laser beam emerging from the non-linear optical crystal element 6 is linearly polarized beam. The fundamental wave laser beam in the form of linearly polarized beam is reflected by the plane mirror 7 and then passes through the non-linear optical crystal element 6 again, whereupon it is changed back into original circularly polarized beam. In this manner, the fundamental wave laser beam which reciprocates through the laser medium 5 is always in the form of circularly polarized beam, and consequently, a spatial hole burning effect is suppressed by a so-called twist mode effect (Applied Optics, Vol. 4, No. 1, January, 1965).
In the conventional laser beam generating apparatus described above, the double refraction amount of the non-linear optical crystal element 6 is adjusted accurately to 90 degrees by adjusting the non-linear optical crystal element 6 in a direction perpendicular to the optical axis LA1, that is, in a direction of the double-sided arrow mark T in FIG. 1, in this manner. However, a very high degree of accuracy is required for such adjustment. If the orientation of the non-linear optical crystal element 6 is displaced upon such adjustment, then the position of an oscillating region of secondary harmonic laser beam is changed. If such displacement actually occurs, then re-adjustment must be performed beginning with the incidence position of pumping laser beam to the non-linear optical crystal element 6, which is very inefficient.
Besides, the concave mirror 3 is not easy to manufacture and is high in cost.